This invention relates in general to vacuum pumping devices and in particular to a new and useful method for testing leaks in vacuum systems.
The present invention concerns a method and arrangement for detecting leaks in walls, where a test gas passing through leaks in a wall is fed over a line to the discharge side of a turbo-vacuum pump and arrives, opposite to the direction of conveyance, in a test gas detector connected to the intake side of the pump.
Suitable arrangements for such a method are known, e.g. from German Pat. No. 1,648,648 and Swiss Pat. No. 519,137. They work on the so-called counterflow principle. Test gas which has passed through a leak, mostly helium, arrives opposite to the direction of conveyance of a turbo-vacuum pump, in the test gas detector connected to its intake side, where it is detected.
A prerequisite for the successful application of this counterflow principle is that a counterflow pump is available which has a compression factor for the test gas which is substantially lower than the compression factor for other gases interfering with the detection of the test gas by the detector. The compression factor for the test gas must be selected so low that it can flow to the detector opposite to the direction of conveyance of the counterflow pump and build up there a measurable partial pressure, the compression factor for the interfering cases, however, should be as high as possible so that their contribution to the signal given off by the detector remains small. Turbo-vacuum pumps are particularly suitable as counterflow pumps because of their compression factor, which depends to a great extent on the molecular weight of the pumped gas.
A drawback of the counterflow principle in known arrangements, however, was the fact that, after each major peak of the partial pressure of the test gas in the detector, a relatively long recovery time was necessary until the full detection sensitivity for following weaker test gas signals could be achieved again. This limitation was mostly due to the fact that the suction power of a forepump was substantially determinant for the test gas being exhausted from the detector, which should be very high for a short recovery time but very low for a high sensitivity of leak detection.
For a conventional leak detector arrangement (see FIG. 1) with a counterflow pump applies the formula: J/Q.sub.leak =E/K.S, where J=the signal proportional to the pressure of the test gas in the detector, e.g. the current of the partial pressure analyzer in ampere;
E=the sensitivity of the detector (size of signal divided by the pressure) indicated in A/mbar, for example; PA1 S=the suction power of the forepump, e.g. in liter per second; PA1 Q.sub.leak =the test gas current through the leak e.g. in mbar 0.1/s; PA1 K=compression ratio of the counterflow pump for the test gas. PA1 K=10, E=10.sup.-4 A/mbar and S=1 l/s; J is thus 10.sup.-5. Q.sub.leak, that is, for the inflow of a test gas of 10.sup.-5 to 10.sup.-10 /mbar l/s it would be necessary to provide an ammeter with measuring ranges of 10.sup.-10 to 10.sup.-15 A. PA1 (a) the required precision ammeter of maximum sensitivity with a measuring range extending over 5 decimal powers is very elaborate; PA1 (b) the system must be evacuated down to the limit range of a mechanical forepump (10.sup.-2 mbar) before the test piece can be connected; such an extensive pre-evacuation is very time consuming, however; PA1 (c) with great leaks, the test gas concentrations in the detector are relatively high, which leads to long recovery periods.
With a given (or desired) sensitivity E, the product K.S. must therefore be as low as possible to obtain a high resolution DELTA Q. But there are limits. K must not be too low for the interfering gases so that a sufficiently low total pressure is maintained in the detector. But the suction power S must not be too low either, otherwise the recovery time will be too long.
Practical values would be, for example:
Such a known arrangement has the following disadvantages: